public class SpecialFlap : Flaps
{
    private readonly List<MyFloop> FloopList;

    public SpecialFlaps()
    {
        this.FloopList = new List<MyFloop>();
    }

    public IReadonlyList<IFloop> Floops => this.FloopList.AsReadonly();

    public IReadonlyList<IFloop> Floops => this.FloopList.OfType<IFloop>().ToList().AsReadonly()
}